Friction stir welding (hereinafter referred to as "stir welding") is a relatively new joining technology disclosed in U.S. Pat. No. 5,460,317, whereby alloys such as 7075 aluminum and various forms of aluminum/lithium previously considered to be unweldable alloys, can be welded to provide joints having minimal change to the structural properties of the parent materials and extremely low dimensional distortion. Unlike fusion welding where portions of each piece being welded are melted and then resolidified to form a new and different material structure, stir welding is a solid state process. Joining is accomplished by stirring and intermixing plasticized material from each part across the joint boundary. Heating and mixing is accomplished simultaneously in stir welding by employing a stepped cylindrical tool having a head pin or probe surrounded at its root end by an annular shoulder. The tool is rotated at a precise computer controlled speed and pressed against the work piece at the joint with substantial axial force which may be in the range of 12,000 to 20,000 pounds. Friction between the rotating probe and work pieces causes local heating and ultimately plasticizing of the material in close proximity to the probe. Once sufficient heat has been generated the material yields and the probe plunges through most of the thickness of the material until the annual shoulder of the tool contacts the surface of the work pieces. The material adjoining the joint is then blended by moving the spinning tool along the joint centerline at a carefully controlled rate and attack angle between the plane of the annular shoulder and that surface of the work pieces. As the probe and shoulder move relative to the work pieces they simultaneously heat and remove material from the leading surfaces in front of the tool and deposit this material onto the trailing surface. The relative movement of the tool along the joint may be accomplished by moving the tool or by moving the work pieces.
Fuel tanks for space vehicles commonly have thin cylindrical body shells with dome-shaped end covers secured together by welding. The thin walls of the tanks may be reinforced by integral internal grids. These grids are machined in flat tank sections which are then bent to provide laterally curved sections. They are then placed side-by-side and connected together by longitudinal butt welds to form cylindrical tank sections. Such sections are in turn welded together at circumferential joints at their ends, and finally, end domes are welded in place at circumferential end joints to complete a tank. Each end cover normally has an access opening which usually is centered, but may not be, and is of a size, about thirty inches in diameter, adequate to permit personnel to enter the tanks during construction and for inspection. Currently, to achieve minimum weight while maintaining adequate strength, materials such as aluminum/lithium alloys are preferred for fabricating the fuel tanks, but this choice requires that stir welding be used. Since the fuel tanks are large, in the range of about 15 to 27 feet in diameter, and commonly 50 feet or so in length, and have relatively thin walls, in the range of about 3/16 to 5/8 inches, for example, stir welding of circumferential joints is difficult. The final joint securing the second end cover in place, has been a particularly vexing problem since the tank is then completely closed except for the access openings in the end covers, and hence, passage for removal of fixtures is very limited. The problem is compounded by the fact that considerable pressure is exerted by the stir welding probe on the work, and hence the work must be backed by a suitable anvil opposite the probe. This means that an anvil must be inside the tank. It also means that the probe pressure must be resisted by a reactive force acting on the outside of the tank. However, such a reactive force together with the probe pressure may seriously distort and perhaps even partly crush the tank unless they are somehow adequately resisted by structure other than the tank.